Icom 706MKIIG modification for frequency injection


On this page, N6GN and K6PZB are documenting two different methods to modify the Icom IC 706MKIIG for use with an external frequency reference.

The first method described uses direct injection and the second one uses an external phase locked loop. The external PLL method is the same technique that is being used on the more general GPS/DOCXO external PLL project.

Modification for Direct Injection Frequency Control for the IC 706

This method uses an external, tripled, 10 MHz reference. This can be local OCXO, GPS displined OCXO or Rubidium standard.
Pop the covers, release bottom board and snake a RG174 cable underneath the filter board located on the rear, topside of the radio. We installed a SMB chassis connector fitted to the one open, external back plate in the 706. This seems to be unused and was convenient. The coax braid was also soldered to a ground near the halfway point where it exited the filter compartment, prior to running it through to the top side near the crystal compartment.

Coax Layout

Coax, resistor and PIN diode installed

Injection Schematics

Board Layout

The shunt resistor is a through-hole 1/8 watt part (labeled 200k ohms and also 147k ohms, it doesn't matter which) that stands on end and is soldered to the the ground area that goes under L623 can, right above the indicated components. The anode of the PIN diode, a SOT-23 surface mount part, is soldered to its top lead. The coax shielde is soldered to the outside of the Oscillator compartment and only the ceenter conductor of the RF174 comes into the compartment. The cathode of the PIN diode is connected to the indicated spot on the board with a short length of bare wire.

706 schematic, with green line indicating new components

Tripler Schematic

Modification for Icom 706MKIIG external Phaselock

This second method uses external cricuits to generate a correction voltage which tunes the radio's master oscillator to be in phase with an external 10 MHz reference. Although this method works fine for the IC706, it's not general enough to work with the variety of HF-UHF amateur transceivers and external reference signals that are often available. We are working on a more general solution but so you can get an idea of the technique, here is some detail of the process.
A single coax cable needs to be installed, just as for the injection technique. However, rather than providing a complete replacement for the 30 MHz on-board oscillator, as was done for the injection method, the phaselock method simply steers that factory oscillator, either standard or high precision, to be precisely in phase with an external frequency reference. Again, this reference may be OCXO, GPS disciplined OCXO or rubidium standard.
Rather than providing DC current to turn off the on-board oscillator and also the replacement 30 MHz RF signal, the phaselock method "sniffs" a little of the 30 MHz MHz coming from the IC706, compares its phase with the external reference and then generates a correction voltage that is returned on the same coax cable to tune the 706 into phase lock condition.
This second method has a potential advantage. Because the master oscillator is used to derive all radio frequencies, if a replacement reference isn't kept very clean in terms of both phase noise and spurious signals, there is a risk of compromising the signal purity of the transceiver. The phase lock method does not have as much risk in this regard. By design, it can only alter the frequency and phase characteristics at a low rate - within the PLL loop bandwidth. This mitigates the danger of worsening the spectral purity of the basic IC 706 transceiver.